1. Field of the Invention
The present invention relates to a method of forming solder bumps on electrodes or lands provided on the surface of an electronic component such as a printed circuit board or chip.
2. Prior Art
For realizing size reduction and densification of an electronic component, there is known a method of forming numerous electrodes arranged in a matrix on the surface of an electronic component and forming bumps (protruding electrodes) on such electrodes. Furthermore, there is known a method of coating solder paste on the electrodes by screen printing and thereafter heating an electronic component in a furnace of a fellow device, thereby melting the coated solder paste to form the bumps.
The screen printing is basically advantageous in that the solder bumps can be formed cheaply with better work efficiency. However, the screen printing necessarily requires heating a printed circuit board to melt the coated solder paste. During this heating operation of the printed circuit board in the furnace of the reflow device, there is a possibility that molten solder paste flows or spreads so widely that adjacent two solder bumps are integrated into a larger bump. Thus, it is feared that adjacent electrodes will be short-circuited by such undesirable integration or bridging of solder bumps. This problem will be explained in more detail with reference to FIGS. 3A, 3B and 3C.,
FIGS. 3A, 3B and 3C explain steps of a conventional method of forming solder bumps. As shown in FIG. 3A, solder pastes 3--3 are coated on electrodes 2--2 formed on the surface of a printed circuit board 1 using the technology of a screen printing. A reference numeral 4 represents a resist film interposed between adjacent two electrodes 2 and 2. The Unexamined Japanese Patent Application No. HEI 3-20099/1991 discloses an example of a screen printing apparatus for coating such solder pastes 3--3.
Next, the printed circuit board 1 is transferred into the furnace of the reflow device, where each solder paste 3 is heated. The Unexamined Japanese Patent Application No. HEI 2-263570/1990 discloses an example of the reflow device. FIG. 3B shows a reflow condition where the molten solder paste 3 causes a side flow along the surface of the printed circuit board 1. Thus, the solder pastes 3, 3 on adjacent two electrodes 2 and 2 are integrated in a fairly large solder bump as shown in the drawing. Thus melted solder paste 3 will be drawn to one side if there is inside pressure imbalance caused by surface tension. For example, when the right side of the molten solder paste 3 has a lower inside pressure, almost all of the solder paste 3 is drawn to the right side as indicated by a dotted line of FIG. 3B. Such a flowing phenomenon possibly results in the formation of an excessively large bump 3a on one electrode 2 (i.e. the right-hand electrode) as indicated by a solid line in FIG. 3C or formation of a defective bump 3b and a bridge 3c short-circuiting adjacent two electrodes 2 and 2 as indicated by a dotted line in FIG. 3C.
In view of recent needs of realizing high integration and densification of electronic components, a pitch of each electrode 2 tends to be narrowed. Meanwhile, to surely solder a bump on a corresponding electrode of an opponent electronic component, it will be necessary to enlarge the volume of each bump. Under such severe circumstances, the recent mounting operation of electronic components inevitably encounters the above-described problem. The same problem will arise in a case where a bump is formed on an electrode on a wafer chip to form a flip chip.